In vertical continuous annealing furnaces a single strand of cold rolled steel strip passes through several zones for heating, soaking and cooling, to recrystallization anneal and perform associated quenching and overageing treatments. For sheet steel annealing with overageing, the annealing cycle typically lasts 5-10 minutes. Strip speed in these furnaces can be as high as 450 mpm for sheet gauges and 650 mpm for tinplate gauges, as dictated by productivity considerations. The length of the furnace is minimized by passing the strip up and down (sinusoidally) over driven support rolls.
The strip moves through the furnace under tension to ensure good conformance to the driven support rolls, and, in combination with roll contours and steering mechanisms, to prevent excessive lateral strip motion leading to mistracking. The application of tension to the strip at high temperature also pulls out cold rolling shape defects through plastic elongation, the extent of which depends on the tension applied, on the steel's deformation resistance, and on the time during which the tension acts on the steel while it is soft enough Lo be deformed by norma values of strip tension. Conventionally, strip tension inside continuous annealing furnaces is most simply controlled by pulling the strip between entry and exit bridles to generate the uniform tension profile. Strip tension can be controlled locally along the furnace by regulating the speeds of individual rolls relative to the strip speed, to step tension up or step tension down to appropriate levels. This procedure will be illustrated below.
Strip tension may also be regulated in discrete zones by using bridles inside the furnace. A bridle is a combination of two or more juxtaposed rolls positioned so as to maximize surface contact between the strip and at least one of the rolls, the latter being a driven roll. In these conventional schemes, tension is regulated at predetermined levels as measured by load cells, which provide a measure of the vertical or horizontal force (e.g., total load) on various support rolls. The appropriate total load used in a particular furnace section depends on strip cross-section (width and thickness), strength (depending on temperature, state of recrystallization and chemical composition), and the need for elongation flattening. The load is limited by the need to prevent creasing, over-necking (the width reduction associated with elongation) and strip breaks. The soaking section is the most critical area for tension control, because the yield strength of the strip is lowest there, typically about 1,000 psi for ultra-low carbon steel at 850.degree.-900.degree. C., making it most susceptible to tension effects.
The range of total load required in a furnace which processes a wide range of strip cross-sections and grades (composition and annealing temperature) makes precise control at the low end of the range difficult because the "dead band" of the best load cells, typically .+-.1 percent of full rated load, represents a large fraction of the total load needed for small cross-sections and soft grades. Harmonic strip flutter also causes actual strip tension fluctuations which broaden the band of uncertainty in load cell measurements. The accuracy of load cell regulation is further limited by the difficulty in distinguishing small changes in strip load in a total load cell signal imposed by strip load and roll weight.